1. Field of the Invention
This invention relates to a field-effect transistor (FET), and more particularly to a FET having a channel formed on a homojunction interface thereof.
2. Description of the Prior Art
In recent years, high-speed performance transistors have been used frequently for supercomputers, and communication apparatus using higher frequencies. Of these transistors, high-speed performance FETs using compound semiconductors, such as GaAs and InP as the basic material, are used. GaAs and InP exhibit high electron mobilities at normal temperature, as compared with Si or the like. FIG. 3 shows a conventional high-speed type FET, which is called a Schottky junction-type FET, formed on a GaAs substrate. This FET is also called a metal-semiconductor field-effect transistor (MESFET).
In FIG. 3, an n-type GaAs layer 32 is formed on a semi-insulating GaAs substrate 30. A Schottky gate electrode 34 is formed on the n-type GaAs layer 32 through an i-type GaAs layer 33. This layer 33 serves to prevent the Schottky barrier from lowering. Reference numerals 35 and 36 respectively designate an n.sup.+ -type GaAs source region and an n.sup.+ -type GaAs drain region. On the respective regions 35 and 36, a source electrode 37 and a drain electrode 38 are formed in ohmic contact with these regions. The MESFET of this kind has a channel 39 formed in the n-type GaAs layer 32. A drain current, which flows in the channel 39, is cut off by a depletion layer. This is because the depletion layer extends from the Schottky gate electrode 34 into the channel 39 in accordance with a gate voltage applied to the Schottky gate electrode 34. If the depletion layer does not reach the channel 39, the drain current continues to flow.
Conventionally, MESFETs are required to enhance the current-driving capacity in order to drive a load connected to the drain electrode 38 at a high speed. To enhance such a current-driving capacity, the gate length must be shortened. However, when the gate length is shortened, the drain current leaks from the channel to the substrate side. As a result, a so-called short-channel effect occurs, in which the threshold voltage (Vth) fluctuates and the current-driving capacity decreases. Specifically, the short-channel effect arises when the gate length is shortened with the channel thickness unchanged causing a significant effect of the drain region with respect to the gate signal. In accordance with a well-known scaling rule, when the gate length is shortened to 1/2, the channel thickness also becomes 1/2.
Thus, the short-channel effect can be avoided. However, when this size-reduction is made with the impurity concentration of the channel unchanged, the threshold voltage (Vth) decreases. To prevent this, the impurity concentration of the channel must be enhanced to four times the original level. However, when the impurity concentration of the channel is increased, this causes the carrier mobility in the channel to be lowered. As a result, high-speed operation of the MESFET cannot be obtained.
On the other hand, another GaAs type FET is known. Namely, a so-called HEMT (high electron mobility transistor) is available. The HEMT has a channel formed by generation of two-dimensional electron gas within an i-type GaAs layer having substantially no impurities. In this channel, electrons can transit with a high mobility. The HEMT was disclosed in Japanese Patent Publication No. 61-49476. FIG. 4a shows an example of the HEMT. In FIG. 4a, an i-type GaAs layer 43 and an n-type AlGaAs layer 42 are grown one after another on a GaAs substrate 40. On the layer 42, a Schottky-gate electrode 44 is formed. Further, a source region 47 and a drain region 48 are formed on both sides of the gate electrode 44.
FIG. 4b is a potential diagram of a cross section taken along line A--A' of the HEMT shown in FIG. 4a. A potential well is generated by the difference between electron affinities of the i-type GaAs layer 43 and the n-type AlGaAs layer 42 at the hetero interface therebetween. The potential well stores electrons so as to generate two-dimensional (2-D) electron gas, as shown in FIG. 4b. The 2-D electron gas forms a channel 49 within the i-type GaAs layer 43 as shown in FIG. 4a. However, an interface level is present in the i-type GaAs layer 43 having the hetero interface. Thus, electrons are captured by the hetero interface which functions as a trap. As a result, the number of electrons that can move freely is limited. Therefore, the formation of a high-electron-density channel is inevitably limited. Consequently, it is substantially impossible to drive a load of large capacity at high speed with conventional HEMTs.
Moreover, it is known that a capture level, which is called a Dx center, is formed within an n-type AlGaAs semiconductor. Thus higher the impurity concentration in the AlGaAs, the more significant the generation of the capture level. Further, the operations of a FET with a capture level vary depending on temperature. Thus, a temperature change causes the trap to free the captured carriers (electrons in the case of an n-type) or to capture the free carriers (electrons). As a result, the conventional HEMTs have problems in that the threshold voltage (Vth) of the channel thereof fluctuates due to the temperature-dependent operation of the capture level. If the impurity concentration in the n-type AlGaAs layer is lowered, generation of capture level can be prevented. However, the electron density in the channel 49 formed on the i-type GaAs layer is inevitably lowered.
As described above, the conventional FETs have problems in terms of high-speed performance. This is because higher carrier mobilities and higher impurity concentrations could not be achieved simultaniously.